1. Field of the Invention
The present invention relates to a method of air/fuel ratio control for an internal combustion engine.
2. Description of Background Information
In order to reduce the level of exhaust gas pollutants and improve the fuel consumption, etc. of an internal combustion engine, it is known to employ an oxygen concentration sensor to detect the concentration of oxygen in the engine exhaust gas, and to execute feedback control of the air/fuel ratio of the mixture supplied to the engine such as to hold the air/fuel ratio at a target value. This feedback control is performed in accordance with an output signal from the oxygen concentration sensor.
One type of oxygen concentration sensor which can be employed for such air/fuel ratio control functions to produce an output which varies in proportion to the oxygen concentration in the engine exhaust gas. Such an oxygen concentration sensor has been disclosed for example in Japanese patent laid-open No. 52-72286, which consists of an oxygen ion-conductive solid electrolytic member formed as a flat plate having electrodes formed on two main faces, with one of these electrode faces forming part of a gas holding chamber. The gas holding chamber communicates with a gas which is to be measured, i.e. the exhaust gas, through a lead-in aperture. With such an oxygen concentration sensor, the oxygen ion-conductive solid electrolytic member and its pair of electrodes function as an oxygen pump element. By passing a flow of current between the electrodes such that the electrode within the gas holding chamber becomes a negative electrode, oxygen gas within the gas holding chamber adjacent to this negative electrode becomes ionized, and flows through the solid electrolytic member towards the positive electrode, to be thereby emitted from that face of the sensor element as gaseous oxygen. The current flow between the electrodes is a boundary current value which is unaffected by variations in the applied voltage and is proportional to the oxygen concentration within the gas under measurement. Thus, by sensing the level of this boundary current, it is possible to measure the oxygen concentration within the gas which is under measurement. However if such an oxygen concentration sensor is used to control the air/fuel ratio of the mixture supplied to an internal combustion engine, by measuring the oxygen concentration within the engine exhaust gas, it will only be possible to control the air/fuel ratio to a value which is in the lean region, relative to the stoichiometric air/fuel ratio. It is not possible to perform air/fuel ratio control to maintain a target air/fuel ratio which is set in the rich region. An oxygen concentration sensor which will provide an output signal level varying in proportion to the oxygen concentration in engine exhaust gas for both the lean region and the rich region of the air/fuel ratio has been proposed in Japanese patent laid-open No. 59-192955. This sensor consists of two oxygen ion-conductive solid electrolytic members, each formed as a flat plate, and each provided with a pair of electrodes. Two opposing electrode faces, i.e. one face of each of the solid electrolytic members, form part of a gas holding chamber which communicates with a gas under measurement, via a lead-in aperture. The other electrode of one of the solid electrolytic members faces into the atmosphere. In this oxygen concentration sensor, one of the solid electrolytic members and its pair of electrodes functions as an oxygen concentration ratio sensor cell element. The other solid electrolytic member and its pair of electrodes functions as an oxygen pump element. If the voltage which is generated between the electrodes of the oxygen concentration ratio sensor cell element is higher than a reference voltage value, then current is supplied between the electrodes of the oxygen pump element such that oxygen ions flow through the oxygen pump element towards the electrode of that element which is within the gas holding chamber. If the voltage developed between the electrodes of the sensor cell element is lower than the reference voltage value, then a current is supplied between the electrodes of the oxygen pump element such that oxygen ions flow through that element towards the oxygen pump element electrode which is on the opposite side to the gas holding chamber. In this way, a value of current flow between the electrodes of the oxygen pump element is obtained which varies substantially in proportion to the oxygen concentration of the gas undler measurement, both in the rich and the lean regions of the air/fuel ratio.
When such an oxygen concentration sensor which produces an output varying substantially in proportion to oxygen concentration is used for air/fuel ratio control, then in the same way as for a prior art type oxygen concentration sensor whose output is not proportional to oxygen concentration, a basic value for air/fuel ratio control is established in accordance with engine operating parameters relating to engine load (e.g. the pressure within the intake pipe, etc.). Compensation of the basic value with respect to a target air/fuel ratio is performed in accordance with the output from the oxygen concentration sensor, to thereby derive an output value; and the air/fuel ratio of the mixture supplied to the engine is controlled by this output value.
However normally when such an oxygen concentration sensor is employed, which produces an output varying in proportion to oxygen concentration, variations in the detector characteristic of the sensor will occur as time elapses, as well as deterioration of the sensor. As a result, the accuracy of correspondence between basic value which is set by using the oxygen concentration sensor and a target air/fuel ratio will be reduced, so that errors will arise. One method which could be envisaged to counteract this is to compute compensation values for compensating errors in the basic value, in addition to the output from the oxygen concentration sensor, and storing these compensation values as data in memory locations which are respectively determined in accordance with the specific engine operating region at the time of computing the compensation value. When computation of the output value is to be performed, in this case, the appropriate compensation value corresponding to the current operating condition of the engine would be obtained by searching the stored data, and the compensation value thus obtained used to compensate the basic value. However in the case of a multi-cylinder internal combustion engine, there will be some differences between the respective amounts of intake air supplied to the various cylinders, even if the operating conditions of all of the cylinders are otherwise identical. This is due to such factors as component accuracy and variations in intake pipe shape. Non-uniformity will thereby arise between the air/fuel ratios of the mixture supplied to the respective cylinders. Thus, if the aforementioned compensation value is computed on the basis of the output from an oxygen concentration sensor, it may not be possible to obtain a desired improvement in exhaust pollution elimination effectiveness.